In a typical cellular radio system, mobile user equipment (UE) communicates via a radio access radio network (RAN) to one or more core networks. User equipment (UEs) comprises various types of equipment such as mobile telephones (also known as cellular or cell phones), lap tops with wireless communication capability, personal digital assistants (PDAs) etc. These may be portable, hand held, pocket sized, installed in a vehicle etc and communicate voice and/or data signals with the radio access network.
The radio access network covers a geographical area divided into a plurality of cell areas. Each cell area is served by at least one base station, which may be referred to as a Node B. Each cell is identified by a unique identifier which is broadcast in the cell. The base stations communicate at radio frequencies over an air interface with the UEs within range of the base station. Several base stations may be connected to a radio network controller (RNC) which controls various activities of the base stations. The radio network controllers are typically connected to a core network.
An example of a radio access network is the Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN). UMTS is a third generation mobile communication system and essentially relates to a wide band code division multiple access (W-CDMA) system.
W-CDMA technology allows simultaneous communication between user equipment and a plurality of base stations using a common frequency band. Signals occupying a common frequency band are discriminated in the receiving base station through spread spectrum CDMA waveform properties based on the use of a high speed, pseudo-noise (PN) code. These high speed PN codes are used to modulate signals transmitted from the base stations and the user equipment. A transmitting station uses a PN code to produce signals that can be separately demodulated at a receiving station. In CDMA therefore a user equipment unit need not switch frequency when handover of the connection is made from one cell to another. Thus, a destination cell can support a connection to the user equipment at the same time as the original cell continues to provide service. Since the UE is always communicating through at least one cell during hand over, there is no disruption to the call.
There are several interfaces defined in the UTRAN. In some instances a connection involves both a serving or source RNC (SRNC) and a target or drift RNC (DRNC), with the SRNC controlling the connection but with one or more diversity legs of the connection being handled by the DRNC. UMTS has both common transport channels and dedicated transport channels. Common transport channels include the random access channels (RACH), the forward access channel (FACH), the common packet channel (CPCH) and the downlink shared channel (DSCH). Dedicated transport channels include the dedicated channel (DCH). These transport channels are described for example in one or more of the following specifications:                ETSI TS 125.211 Version 3.12.0 “Physical Channels and Mapping of Transport Channels on to Physical Channel (FDD)”;        ETSI TS 125.221 Version 3.11.0 “Physical Channels and Mapping of Transport Channels on to Physical Channel (TDD)”;        ETSI TS 125.331 Version 3.15.0 “Radio Resource Control (RRC) Protocol Specification.        
When switching from a dedicated to a common channel in UMTS, the UE is assigned a temporary identifier known as C-RNTI (Cell Radio Network Temporary Identifier). This temporary identifier C-RNTI is specific to a cell and is valid in the cell where the UE uses resources. The C-RNTI is used to distinguish between different user equipment in that cell.
When switching from dedicated channels to common channels, the network can assign the radio resources to be used only if the UE is in a cell under control of the serving SNC (SRNC). In other words, if the UE is in a cell under the control of a drift RNC (DRNC) rather than SRNC, it is not possible for the network to assign the radio resources to be used when switching from dedicated channels to common channels. This results in longer signalling sequences between the UE and the network as well as additional delay when switching from dedicated channels to common channels. The UE itself must obtain the temporary identifier C-RNTI by additionally signalling between the UE and UTRAN before continuing to communicate with the network. Furthermore, the lack of information regarding which range of resources to use means that the UE needs to detect and read information about the range of resources and the broadcast channel in the cell before accessing the cell. This again incurs additionally delay when switching from dedicated channels to common channels.